For some reason I cannot post an answer. It seems that word fallacy could be suitable.
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JeneaJun 18 '12 at 17:07

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I can't help but think that the fact that this question has 26 answers is indicative that it is kind of problematic.
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LessPop_MoreFizzJun 19 '12 at 2:08

@LessPop_MoreFizz: I don't think it is problematic. the thing is, the person is looking for metaphors and there must be at least dozens of it. Usually one person doesn't know all of them, so each person gives as answer one of their own. It is impossible to get a single right answer to this (although it is possible to get many wrong answers)
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TamesJun 19 '12 at 16:24

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@LessPop_MoreFizz: language problems are not like 1+1=2!
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TamesJun 19 '12 at 16:48

If you want a proverb, it could be "Not all that glitters is gold", although what is not good is not necessarily bad.
Perhaps a good expression could be "a wolf in sheep's skin", as this is exactly a bad thing disguised as good.

For your second example "the wrong tool for the job" is a fairly common phrase. It means that what you're trying to use is, in fact, a useful tool, but it's not the right one to use on this particular job. (The tool doesn't have to be literally a mechanical or hand tool.)
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JLGJun 18 '12 at 13:14

In many cases, the relatedly devlish phrase, "the devil is in the details," could be appropriate. For the second bullet point: "That screwdriver appeared to be about the right size for what I needed. Unfortunately, the devil is in the details and that metric screwdriver didn't fit my half inch screws."
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wrosecransJun 19 '12 at 2:56

The term is sometimes used to describe something disappointing – something that doesn't live up to expectations.

(Maybe you were wanting something much worse than being merely disappointing, but bad is a rather vague word, so it's hard to know for sure. If that's the case, though, I'm sorry about this hollow bunny of an answer.)

I like the answer "Poisoned Chalice", from @Jasper Loy above. You might also consider, when talking about a situation instead of a thing, "primrose path". A path or way that is pleasant to walk but leads to disaster.

"No all that glitters is gold" - people sometimes chase after stuff because it looks like it's what they want, even when it isn't. Easily applicable to stuff like the "Cult of New" where people want the shiny new technology.

For furniture, you might use "rickety" to describe something that would break if you sat on it.

If you're talking just about tools, I suppose you could use "Golden Hammer". It means that you have a tool you like so much that you use it for everything.

"Thin Reed" - something that might seem sturdy, but would break if leaned on it.

A common variant is "about as much use as a Chocolate fireguard". However, I avoided that phrase in my answer to this question, because I feel it's not really what the questioner is looking for.
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Dominic CroninJun 18 '12 at 20:40

"Pandora's Box", "A Can of Worm", and "All Hell Breaks Loose" have nothing to do with something that appears good at first but turns out not to be good.
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Mr. Shiny and New 安宇Jun 18 '12 at 13:28

If you were Pandora, the box would be good at first same with the can of worms for a Fisherman... each statement has a context with an equal and opposite position via the context of its use.
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JayJun 19 '12 at 0:57

@Jay: if you're a fisherman that can of worms appears just as good or bad whether you've opened it or not. And the idiomatic phrase "a can of worms" has nothing to do with something that seemed like a good idea at the time but isn't now. Nor does the allusion of Pandora's Box.
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Mr. Shiny and New 安宇Jun 19 '12 at 12:23

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@Mr.ShinyandNew安宇 How about eating very spicy, very greasy chorizo on the way in to work?
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shinyspoongodJun 20 '12 at 0:45